Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. In the field of semiconductor metrology, a metrology tool includes an illumination system which illuminates a target, a collection system which captures relevant information provided by the illumination system's interaction (or lack thereof) with a target, and a processing system which analyzes the information collected using one or more algorithms. Metrology tools can be used to measure structural and material characteristics (e.g., material composition, dimensional characteristics of structures and films such as film thickness and/or critical dimensions of structures, overlay, etc.) associated with various semiconductor fabrication processes. These measurements are used to facilitate process controls and/or yield efficiencies in the manufacture of semiconductor dies.
Bright field (BF) and dark field (DF) metrology modalities may be used to measure specular or quasi-specular surfaces such as semiconductor wafers. BF metrology systems position collection optics to capture a substantial portion of the light spectrally reflected by the surface under inspection. DF metrology systems position collection optics substantially out of the path of the spectrally reflected light such that the collection optics capture light scattered by objects on the surface being measured. Viable metrology systems, particularly systems employing a BF measurement modality, require high radiance illumination and a high numerical aperture (NA) to maximize the defect sensitivity of the system. In general, the defect sensitivity of a metrology system is proportional to the wavelength of the illumination light divided by the NA of the objective. Without further improvement in NA, the overall defect sensitivity of current metrology tools is limited by the wavelength of the illumination source.
In some examples, current metrology systems employ an electrode-based, relatively high intensity discharge arc lamp to generate illumination light. These arc lamps include an anode and cathode to excite a working medium (typically xenon or mercury gas) contained within a chamber of the lamp. An electrical discharge is generated between the anode and cathode to provide power to the excited (e.g., ionized) gas to sustain light emission from the ionized gas during operation of the light source. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current. As a result, state of the art xenon-based arc lamps typically generate light with a color temperature that is limited to approximately 6,000 degrees Kelvin.
In some other examples, laser sustained plasma based light sources have been developed. An exemplary laser sustained plasma system is described in U.S. Pat. No. 7,786,455 assigned to Energetiq Technology Inc. In one example, a xenon lamp is ignited with a high voltage pulse applied through electrodes. Once started, the plasma is continuously sustained with a powerful continuous-wave (CW) laser beam focused to a small spot inside the Xenon gas envelope. However, laser sustained plasma light sources also face significant limitations. The laser sustained plasma develops a temperature gradient across the plasma (i.e., a hot central core surrounded by cooler outer portions) that causes self-absorption; particularly absorption of short wavelength light. As a result, it is difficult to increase the accessible plasma temperature beyond approximately 12,000-15,000 degrees Kelvin. Moreover, an increase in laser power generally results in a larger plasma size that has a diminishing impact on color temperature.
In addition, state of the art light sources that employ electrodes must arrange the long axis of the lamp perpendicular to the gravitational field to achieve an acceptable signal to noise ratio (SNR). Unfortunately, certain metrology system architectures (e.g., broadband spectroscopic ellipsometers) employ a rectangular illumination slit (e.g., polarizer slit) that is not aligned perpendicular to the gravitational field. In these systems, the long axis of the illumination slit and the long axis of the plasma are currently crossed. This results in loss of light, higher source non-uniformity across the polarizer slit, and higher sensitivity to residual wedges in rotating elements, such as a polarizing prism.
In some other examples, light sources employing laser produced plasma have been developed for lithographic applications. An exemplary extreme ultraviolet (EUV) light source is described in U.S. Pat. No. 7,368,741 assigned to Cymer, Inc. In one example, a working medium at low pressure is pre-ionized by a radiofrequency coil, ignited with a focused laser beam, and sustained by a combination of the focused laser beam and electrical discharge to generate a pinch plasma that emits EUV light. Generation of illumination suitable for EUV lithography requires emission along narrow atomic lines, rather than broadband emission. As a result, EUV illumination sources generate plasma in a low pressure (i.e., vacuum) environment to minimize reabsorption and broadening of the desired EUV emission lines. The low pressure environment allows the plasma to expand into a large volume, corresponding to a mean-free-path between collisions. If relatively high pressures were employed (e.g., 0.5 atmosphere, or greater), these sources would fail to generate any useable amount of EUV light. Large plasma volumes are acceptable for an EUV source that focuses on the generation of as much EUV emission along atomic lines as possible. However, in a metrology application, high radiance, broadband radiation is required. The large plasmas generated by systems designed for EUV emission suffer from low brightness and narrow band emission that does not fulfill the requirement for broadband, high brightness illumination in metrology applications.
In some other examples, laser induced break-down of the target is employed for analysis of the target itself. In these examples, a pulsed laser is used to evaporate a small amount of material from a specimen. The resulting plasma radiation is spectrally analyzed to reveal properties of the specimen. This technique is commonly referred to as Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Plasma Spectroscopy (LIPS). In these applications, laser induced breakdown of the target itself is used to gain insight into the chemical composition of the target. However, no consideration is given to the use of the emission from the primary target to illuminate another, secondary target for purposes of analyzing properties of the secondary target based on its interaction with the emitted light.
Existing laser based plasma illumination sources are limited in brightness and reliability for metrology applications. Thus, improved methods and systems for generating and extracting high brightness, broadband light at suitable flux levels are desired.